Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Typically, a transmission has a housing mounted to the vehicle structure, an input shaft driven by an engine crankshaft, and an output driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns.
FIG. 1 schematically illustrates a Dual Clutch Transmission (DCT). Input 20 is adapted for coupling to an engine crankshaft, potentially via a damper assembly that reduces the transmission of engine pulsations. Ring gear 22 is fixedly coupled to a differential to distribute power between two drive wheels. First output pinion 24 is fixedly coupled to first layshaft 26 and meshes with ring gear 22. Second output pinion 28 is fixedly coupled to second layshaft 30 and also meshes with ring gear 22. First friction clutch 32 selectively couples input 20 to solid shaft 34, while second friction clutch 36 selectively couples input 20 to hollow shaft 38 which is concentric with solid shaft 34.
Gears 40 and 42 are supported for rotation about first layshaft 26 and mesh with gears 44 and 46 respectively which are fixedly coupled to solid shaft 34. Coupler 48 selectively couples gear 40 or 42 to first layshaft 26. Gear 50 is supported for rotation about second layshaft 30 and meshes with gear 52 which is fixedly coupled to solid shaft 34. Coupler 58 selectively couples gear 50 to second layshaft 30. When couplers 48 or 58 have coupled one of gears 40, 42, or 50 to the respective layshaft, a power flow path is established between solid shaft 34 and ring gear 22. Each of these different power flow paths is associated with a different speed ratio. When clutch 32 is also engaged, a power flow path is established between input 20 and ring gear 22.
Gears 60 and 62 are supported for rotation about second layshaft 30 and mesh with gears 64 and 66 respectively which are fixedly coupled to hollow shaft 38. Coupler 68 selectively couples gear 60 or 62 to second layshaft 30. Gears 70 and 72 are supported for rotation about first layshaft 26 and mesh with gear 66 and 60 respectively. Coupler 74 selectively couples gear 70 or 72 to first layshaft 26. When couplers 68 or 74 have coupled one of gears 60, 62, 70, or 72 to the respective layshaft, a power flow path is established between hollow shaft 38 and ring gear 22. When clutch 36 is also engaged, a power flow path is established between input 20 and ring gear 22. The speed ratios associated with clutch 36 are interleaved with the speed ratios associated with clutch 32 such that clutch 32 is used to establish odd numbered gear ratios and clutch 36 is used to establish even numbered gear ratios and reverse.
When a driver selects Drive with the vehicle stationary, coupler 48 is commanded to couple gear 42 to shaft 26 while clutch 36 is commanded to disengage. To launch the vehicle, clutch 32 is commanded to gradually engage. Similarly, when Reverse is selected with the vehicle stationary, coupler 74 is commanded couple gear 72 to shaft 26. Then, clutch 36 is commanded to gradually engage to launch the vehicle. When cruising in an odd numbered gear, clutch 32 is engaged. To shift to an even numbered gear, clutch 36 is disengaged (if it was not already disengaged), and either coupler 68 or 74 pre-selects the destination power flow path. After the destination gear is pre-selected, clutch 32 is released and clutch 36 is engaged in a coordinated fashion to transfer power between the corresponding power flow paths and adjust the overall speed ratio.
Clutches 32 and 36 may be either dry or wet friction type clutches. One or more friction plates are fixedly coupled one of the elements while a housing with a pressure plate and a reaction plate is fixedly coupled to the other element. The friction plates are between the pressure plate and the reaction plate. If there is more than one friction plate, they are separated by separator plates that are also fixedly coupled to the housing. When the clutch is fully disengaged, the reaction plate and the pressure plate are spaced apart such that the friction plate can rotate relative to the housing with minimal drag torque. To engage the clutch, an actuator causes a normal force that squeezes the friction plate(s) between the pressure plate and the reaction plate. The torque capacity of the clutch is proportional to the normal force and also proportional to the coefficient of friction. The coefficient of friction may depend upon the relative speeds and on other factors such as the clutch temperature. Ideally, the coefficient of friction varies continuously with changes in relative speed, but some clutch materials depart from this ideal behavior and exhibit a sharp reduction in coefficient of friction between no slip and some slip. If the elements are rotating at different speeds, the clutch exerts torque on each element equal to the torque capacity in a direction tending to equalize the speeds. If the elements are at the same speed, then the clutch transfers as much torque as is applied up to the torque capacity. If the applied torque exceeds the torque capacity, then the clutch slips creating relative speed.
Some clutches use position controlled actuation in which a controller commands the actuator to move to a specified position. The actuator may be linked to the pressure plate and reaction plate through springs such that the clutch normal force may be adjusted by adjusting the actuator position as illustrated in FIG. 2. As the actuator moves through the disengaged region at 80, the normal force is zero. After the actuator position passes a touchpoint 82, the normal force increases in proportion to changes in the actuator position. When the controller commands a change in direction of actuator position, the normal force may remain constant for some distance before changing direction as shown at 84, due to hysteresis. Some clutches may respond to other ways of adjusting the normal force, such as adjusting a hydraulic pressure instead of adjusting an actuator position. These other mechanisms may also be characterized by a touchpoint and hysteresis.